Inert pairs

The tendency of elements of higher atomic number to retain the s electrons as an inert pair is also encountered in Group IV, and in this case it is found that for lead the most stable oxidation state is + 2, achieved by loss of two p electrons. 

The steady trend towards increasing stability of rather than M compounds in the sequence Ge, Sn, Pb is an example of the so-called inert-pair effect which is well established for the heavier post-transition metals. The discussion on p. 226 is relevant here. A notable exception is the organometallic chemistry of Sn and Pb which is almost entirely confined to the state... 

Although both aluminum and indium are in Group 13/III, aluminum forms A1J+ ions, whereas indium forms both In3+ and In+ ions. The tendency to form ions two units lower in charge than expected from the group number is called the inert-pair effect. Another example of the inert-pair effect is found in Group 14/IV tin forms tin(IV) oxide when heated in air, but the heavier lead atom loses only its two p-electrons and forms lead(II) oxide. Tin(II) oxide can be prepared, but it is readily oxidized to tin(IV) oxide (Fig. 1.56). Lead exhibits the inert-pair effect more strongly than tin. 

The inert-pair effect is the tendency to form ions two units lower in charge than expected from the group number it is most pronounced for heavy elements in the p block. 

FIGURE 1.57 The typical ions formed by the heavy elements in Groups 13/III through 15A/ show the influence of the inert pair—the tendency to form compounds in which the oxidation numbers differ by 2. 

Identify which of the following elements experience the inert-pair effect and write the formulas for the ions that they form (a) Sb (b) As (c) Tl (d) Ba. 

Many metallic elements in the p and d blocks, have atoms that can lose a variable number of electrons. As we saw in Section 1.19, the inert-pair effect implies that the elements listed in Fig. 1.57 can lose either their valence p-electrons alone or all their valence p- and s-electrons. These elements and the d-block metals can form different compounds, such as tin(II) oxide, SnO, and tin(IV) oxide, Sn02, for tin. The ability of an element to form ions with different charges is called variable valence. 

Group 13/III is the first group of the p block. Its members have an ns np1 electron configuration (Table 14.5), and so we expect a maximum oxidation number of +3. The oxidation numbers of B and A1 are +3 in almost all their compounds. However, the heavier elements in the group are more likely to keep their s-electrons (the inert-pair effect, Section 1.19) so the oxidation number +1 becomes increasingly important down the group, and thallium(I) compounds are as common as... 

The first three members of this series appear at the bottom of the B subgroups of the periodic groups 4, 3 and 2. They exhibit the so-called inert-pair effect and normally assume oxidation states of -l-4,-l-2 -l-3,-f-1 and -1-2,0 respectively, i.e. differing by two units. The species Hg" ", TF" and Pb are of high energy and... 

The term inert pair is often used for the tendency of the 6s2 electron pair to remain formally unoxidized in the compounds of Pb(n) [and also in the case of T1(I) and Bi(m) etc.]. As discussed above, this tendency can be related to relativity. Figure 59 shows the relativistic and non-relativistic valence orbital energies for Sn and Pb. The relativistic increase of the s-p gap leads to a 6s2 inert pair in the case of Pb. However, the situation is more complex if the local geometry at the heavy atom (Pb) is discussed. There are examples for both, stereochemically inactive and stereochemically active s2 lone pairs. 

A given main group metal typically displays one oxidation state, usually equal to its family number in the periodic table. Exceptions are elements such as T1 (+1 and+3), Pb (+2 and +4), and Sn (+2 and +4) in which the lower oxidation state represents a pair of s electrons not being ionized (a so-called inert pair ). 

The inert pair concept has sometimes been advanced to account for the extra stability of atoms or ions which contain a lone pair of s-electrons (e.g.,... 

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